1. Technical Field
This invention relates to a method and apparatus for refining boron-containing silicon (such as scrap silicon containing boron as a dopant) using an electron beam which can obtain high-purity silicon for use in solar cells, semiconductors, or other devices.
2. Related Art
For the following reasons, it is difficult to remove boron contained in a silicon melt.
1. Boron has low activity, and removal thereof by vaporization is difficult (boron has a low vapor pressure, so simple vacuum removal cannot be performed).
2. The segregation coefficient of boron is close to 1, so solidification refining is not a viable method.
There have been a number of research reports pertaining to removal of boron from a silicon melt by slag treatment, plasma treatment, and treatment which is a combination of these two treatment methods. Examples of plasma treatment methods which have been proposed include treatment by a weakly oxidizing transferred arc plasma using carbon dioxide gas, treatment in which O2 or H2O is added to a nontransferred arc plasma, treatment in which induction melting and a nontransferred arc plasma are combined and H2O is added to the plasma, and a method in which water vapor is added to a nontransferred argon arc plasma so as to remove boron. For example, Japanese Published Unexamined Patent Application Hei 10-245216 describes blowing a mixture of argon gas and water vapor at the surface of a melt of silicon which was melted using a plasma torch and oxidizing boron in the melt.
In these treatment methods which have been proposed in the past, boron is removed from a silicon melt by vaporizing a compound (a boron oxide) having a high vapor pressure which is formed by a reaction between boron and added O2, H2O, or other substance which is added to the melt. However, in these methods, due to the nature of the heating method used to heat the silicon, molten silicon in the silicon melt is violently stirred while reacting with the substance which is added to the melt, so silicon itself also ends up oxidizing. As a result, these methods have the problem that the yield of high-purity silicon obtained by refining ends up decreasing.